Blow molding processes for forming PET containers are well known in the art. PET plastic containers have replaced or provided an alternative to glass containers for many applications. However, few food products that must be processed using pasteurization or retort are available in plastic containers. Pasteurization and retort methods are frequently used for sterilizing solid or semi-solid food products, e.g., pickles and sauerkraut. The products may be packed into the container along with a liquid at a temperature less than 82° C. (180° F.) and then sealed and capped, or the product may be placed in the container that is then filled with liquid, which may have been previously heated, and the entire contents of the sealed and capped container are subsequently heated to a higher temperature. As used herein, “high-temperature” pasteurization and retort are sterilization processes in which the product is exposed to temperatures greater than about 80° C.
Pasteurization and retort differ from hot-fill processing by including heating the filled container to a specified temperature, typically greater than 93° C. (200° F.), until the contents of the filled container reach a specified temperature, for example 80° C. (175° F.), for a predetermined length of time. That is, the external temperature of the hot-filled container may be greater than 93° C. so that the internal temperature of a solid or semi-solid product reaches approximately 80° C. Retort processes also involve applying overpressure to the container. The rigors of such processing present significant challenges for the use of plastic containers, including containers designed for use in hot-fill processing. For example, during a retort process, when a plastic container is subjected to relatively high temperatures and pressures, the plastic container's shape will distort. Upon cooling, the plastic container generally retains this distorted shape or at least fails to return to its pre-retort shape.
Prior art base designs tend to deform significantly when their plastic blow-molded containers are exposed to a thermal process comprising, for example, heating the container to a temperature of from about 98° C. to about 127° C. for about 10 to about 40 minutes followed by cooling to about from 25° C. to about 37° C. in from about 10 minutes to about 30 minutes. Such temperatures are typical for hot fill applications as well as sterilization applications such as retort and pasteurization. The deformation typically manifests in a lean to the container—sometimes as much as from 3 to 5°. The perpendicularity of a plastic blow-molded container is important for the ability to properly apply a label, shelf appearance and the ability to stack containers on top of each other. Base deformation will also increase the risk of fracturing barrier layers applied to any food container needing improved oxygen performance. Accordingly, there is a need to provide plastic containers having base designs that can withstand such extreme conditions associated with pasteurization and retort processing.